1. Field of the Invention
This invention relates to a tone detector circuit of the type employing the period detection technique, and more particularly to such a circuit which is of special utility for tone receivers used in telecommunications.
2. Description of the Prior Art
A tone receiver is an electronic device used in telecommunications systems to convert a Dual Tone Multiple Frequency (DTMF) signal into a multiple digit binary code. The format of a DTMF signal consists of high and low tones signal bands (697-941 Hz., and 1209-1633 Hz.). Each band carries a selected one tone of a predetermined set of four tone signals unique to the respective band. That is, there is simultaneous presence of two tones, with one in each band and with each such one tone in each band being a selected one of four possible tones for the band. The DTMF signal system is sometimes referred to as a "2-of-8" signal system.
A tone detector is a device for detecting which of the pedetermined set of tones in one of these bands is present. Tone detectors of the period detection type measure the interval between zero-crossings of the analog input signal in the band as a multiple of a crystal-controlled clock signal. The detection of a tone is provided by a set of period range decoder gates which detect period counter output states spanning ranges of frequencies about the precise frequencies of the set of tones in the band.
Typically, axis crossing signals are detected by a Schmidt-Trigger circuit.
The choice of detection bandwidths (i.e. the range of period counter output states detected by the period range decoder gates) is a compromise. Narrowing detection bandwidth increases the detector's capability to reject tones deviating from the precise frequencies of valid tones. Widening the detection bandwidth increases toleration of distortion of a valid tone by such causes as cross modulation from the other band, white noise, spikes, dial tone, and power line pickups. The quality of rejection of tones deviating from the precise frequencies of a valid tone is sometimes referred to as the "Voice Guard" quality of the detector. The reason for this use of the term is that a common source of invalid tones is the inadvertent simulation of a valid tone by a voice signal.
Prior to the present invention the above described compromise in detection bandwidth has been effected by choosing detection bandwidths which were relatively large and resulted in undesired degradation of the voice guard quality of a tones detector. More specifically, the detection bandwidths of the frequency range detector gates had to be sufficiently large to reliably operate under conditions of "jitter" in the zero-crossings signal from the Schmidt-Trigger axis-crossing detector circuit. These conditions of jitter sometimes occurred as a result of noise and amplitude variations caused by the sources mentioned in the preceding paragraph.
It is also important that a tone detector operate without any unnecessary delay in processing a tone burst. Under U.S. telecommunications practices a DTMF signal burst must be rejected if less than 20 milliseconds in duration and must be detected if more than 40 milliseconds in duration. On the other hand, in the lower frequency band of a DTMF signal system a failure to properly process one period of a received tone can cause a delay amounting to the better part of 5 milliseconds. The problem of quick response in processing a signal is particularly difficult if a tone burst is preceded by a nonvalid tone conditions.
Accordingly, among the objects of the present invention are the provision of:
1. A tone detector having smaller detection bandwidths than heretofor possible with prior art apparatuses.
2. A tone detector having improved voice guard qualities.
3. A tone detector in accordance with the preceding objective which achieves the improved voice guard quality with minimal degradation of speed of response.
4. A tone detector capable of operating in the presence of conditions of jitter of zero-crossings signals, without significant degradation of detection quality.
5. A tone detector which retains improved performance over a wider signal dynamic range.